There are numerous systems where it is required to generate a specific frequency or to select a specific narrow frequency band rapidly by electrical tuning. A selectable narrow band generator finds use in transmitters for electronic countermeasures and secure communications systems. Other systems may require a tunable filter to select a narrow frequency band as in a receiver to eliminate signals at all frequencies other than the band of interest. In both instances, the frequency of interest must be rapidly changeable for such applications as sampling, frequency hopping, encription and other similar uses.
Some circuits for generators or electronically tunable filters for selecting one of a plurality of frequencies involve closed loop feedback systems which provide very accurate frequency selection but are slow to lock on to frequency. Other circuits involve open loop systems which shift frequency rapidly but do not provide for accurate frequency selection.